I. Field
This disclosure is related to power supply circuits for powering lamp bulbs.
II. Description of Related Art
Display technology (e.g., for use in computer and entertainment display devices) continues to advance, as generally is the case with consumer and business electronics. Display devices (such as digital display projectors, flat panel displays, plasma displays, cathode-ray-tube (CRT) displays, etc.) continue to improve in the quality and resolution of the images they display. A wide variety of such display systems are available from InFocus Corporation of Wilsonville, Oreg., the assignee of the present application.
Projection display devices, such as those manufactured by InFocus, include an optical subsystem for displaying images (e.g., still images or video). Such optical subsystems typically include an illumination source (e.g., a high pressure mercury lamp) for generating light to project such images. The illumination source (lamp) is powered (driven) by a lamp driver circuit. Current lamp driver circuits have certain drawbacks, however.
One drawback of current lamp driver circuits is that a tradeoff is made in their design process due, in part, to that fact that most lamp bulbs (lamps) currently used in displayed devices are short arc lamps. In order to improve the life of such lamps, it is desirable that the output capacitors of the lamp driver circuit used to drive the lamp be relatively small in order to reduce the amount of transient current that is delivered to the lamp from the driver circuit. However, the amount of ripple current (e.g., resulting from conversion of alternating current power to direct current power) should also be reduced to prevent arc jump (which may damage the lamp) and flicker (which may adversely effect the quality of projected images). Current approaches use passive and inductive filtering to reduce ripple current. The use of such filtering conflicts with the goal of reducing the size of the lamp driver circuit's output capacitors. Therefore, in such approaches, a trade off is typically made between reducing ripple current and reducing the size of the lamp driver circuit output capacitors.
Another drawback of current lamp driver circuits is the overall cost of such circuits. Current approaches for implementing lamp driver circuits utilize two active converters, a front end converter, which may be termed a power factor converter, and a back end converter, which converts the power provided by the front end converter to power (typically direct-current (DC) power) that is usable for illuminating (driving) the lamp. A typical configuration of a current lamp driver circuit includes a boost converter for the front end converter that both rectifies power from an alternating-current (AC) power source (e.g., 120V residential AC power) and steps-up that rectified power to a high voltage (e.g., 400-500V) in order to adjust the power factor (e.g., the strain on the AC power source) and/or adjust the effective power consumption of the lamp driver circuit.
In such a configuration, the back-end converter is typically implemented as a buck converter that steps down the high voltage produced by the front-end converter to a voltage that is usable by the lamp (e.g., 40-50V). Because the front-end converter and back-end converter are both active circuits that include active components and control circuits (e.g., pulse-width modulation controllers), such approaches may be expensive. Further, such approaches also suffer from the trade off between reducing output capacitor size and the reduction of ripple current. Based on the foregoing, alternative approaches for implementing lamp driver circuits are desirable.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.